Key telephone hold arrangement

Abstract

There is disclosed a line circuit for establishing a hold condition on a communication line, extending between a switching machine and a key telephone station, during an open switching interval (OSI) when no current flows on the communication line. The control signal required to enable the hold circuit is arranged to be time dependent. For a fixed time interval, which bridges the OSI period, after the telephone station answers a ringing line a hold condition is enabled (402) exclusively by a control signal on the A lead from the station. This time dependent hold enable arrangement permits a hold capability during an OSI condition. After this time interval elapses (403) a hold condition is enabled by (404) a joint signal condition on the A lead and a current continuity condition on the communication line.

We claim: 
     
       1. A line circuit for use in a key telephone system for controlling a hold function on a communication path extending between a switching machine and a telephone station, said line circuit adapted for station connect and station disconnect signaling over said communication path and over an A-lead path between said line circuit and said station, said line circuit including means (509, 510, 511) jointly responsive to a station disconnect signal on said A-lead path and a station connect signal on said communication path for applying a hold impedance across said communication pair   characterized in that   means (502, 504, 508, 509, 510, 511) effective during a fixed time interval following a station connect answer occurring during a ringing signal from the switching machine for applying a hold impedance across the communication path solely in response to a station disconnect signal over one of said paths.   
     
     
       2. The invention of claim 1 characterized in that   said means for applying is responsive solely to a non-ground signal received over said A-lead path.   
     
     
       3. The invention of claim 2 characterized in that   said hold applying means includes   means (504) for detecting a ringing station answer signal on said communication path, and   means (502) operable in response to said ringing station answer detecting means for establishing the time interval.   
     
     
       4. The invention of claim 3 characterized in that   said time interval establishing means includes   an interval terminating means (512, 513, 516) enabled by said ringing station answer detecting means for defining the termination of the time interval and   a flip-flop means (502) set by said ringing station answer detecting means and reset by said interval terminating means.   
     
     
       5. A line circuit for use in a key telephone system for controlling the hold function on a communication pair extending between a switching machine and a telephone station, said line circuit including means (IC1A) for detecting signals on an A-lead between said line circuit and said station,   means (IC1A, 103) responsive to signals on said communication pair for generating a control signal and   means (510, 511) jointly responsive to said A-lead signal detecting means and said generating means control signal for applying a hold impedance across said communication pair   characterized in that   said line circuit further includes   means (502, 504, 508, 509) interposed between said communication pair signal detection means and said hold applying means and effective during a fixed time interval following a station answer occurring during a ringing signal from the switching machine for applying a signal for the extent of said interval to said hold applying means equivalent to the control signal from said generating means whereby said hold applying means is rendered unresponsive to the generating means control signal during said fixed time interval.   
     
     
       6. The invention of claim 5 characterized in that   said generating means includes   means (103) connected in shunt across said communication pair for detecting momentary station transition signals occurring whenever said telephone station makes an on-hook to off-hook or off-hook to on-hook transition.   
     
     
       7. The invention of claim 5 characterized in that   said line circuit further includes   means (504) for detecting a ringing station answer signal on said communication pair and   means (502) operable in response to said ringing station answer detecting means for establishing said fixed time interval.   
     
     
       8. The invention of claim 7 characterized in that   said time interval establishing means includes   an interval terminating means (512, 513) enabled by said ringing station answer detecting means for defining the termination of the time interval and   a flip-flop means (502) set by said ringing station answer detecting means and reset by said interval terminating means.   
     
     
       9. A line circuit for use in a key telephone system wherein said line circuit is adapted to control communication connections between a central switching machine and a telephone station, said line circuit comprising: means (IC1A) for detecting the presence or the absence of a ground signal on an A-lead from said telephone station,   means (IC1A, 103) for detecting a station answer occurring during a ringing signal on a communication pair between said switching machine and said station,   characterized in that   said line circuit further includes   means (502, 504) for establishing a time interval in response to a signal from said ringing station answer detecting means and   means (509, 510, 511) enabled during said established time interval means for applying a hold impedance across said communication pair in response to a signal from said A-lead detecting means indicating a transition from the presence to the absence of a ground signal on said A-lead.   
     
     
       10. The invention of claim 9 characterized in that   said established time interval means includes   an interval terminating means (512, 513) enabled by said ringing station answer detecting means for defining the termination of the time interval and   a flip-flop means (502) set by said ringing station answer detecting means and reset by said interval terminating means.   
     
     
       11. A method of enabling a hold condition on a telephone line extending between a switching machine and a line circuit of a telephone station comprising the steps of providing a signal indicating the answering of a ringing station telephone line,   providing a signal indicating the transition from a grounded signal to an ungrounded signal on an A-lead extending between said line circuit and said telephone station and   applying a hold impedance across said communication pair in response to said ringing station answering signal followed within a fixed time interval by said A-lead signal.   
     
     
       12. The invention of claim 11 wherein said applying step comprises the steps of storing said ringing station answering signal for a fixed timed interval and   controlling the connection of said hold impedance in response to both said stored ringing station answering signal and said A-lead signal.   
     
     
       13. The invention of claim 12 wherein said applying step further comprises the steps of providing a signal a fixed time interval after said ringing station answering signal and   clearing said stored ringing station answering signal in response to said fixed time interval signal.